1. Field of the Invention
The present invention relates to permanent magnet synchronous machines and, more particularly, to a method for smoothing the torque output of quasi square wave back EMF permanent magnet (PM) synchronous machines.
2. Description of the Background
Surface mount permanent magnet (PM) AC machines have concentrated, full pitch, stator phase windings and uniformly magnetized, approximately full pitch rotor magnets.
When the excitation voltage is removed from the outgoing stator phase winding of PM-AC machine, and the excitation voltage is applied to the next incoming phase winding, there exists a difference between the rate at which the current falls in the outgoing phase winding and the rate of increase in the incoming phase winding. This differential results in a momentary torque notch (or depression) in the torque of the PM machine.
Certain applications for PM machines require smooth torque characteristics. Hence, various methods have evolved for controlling both voltage source and current source drives in order to reduce the above-described torque notch.
Several efforts have resulted in potential solutions for the torque ripple or notch problem in three phase square wave PM machines.
For instance, in Le-Huy et al., "Minimization of Torque Ripple in Brushless DC Motor Drives," IEEE Trans. on IA, Vol. IA-22, No. 4, pp. 748-755 (July/August 1986), modulation of the DC link current was proposed to compensate for back emf waveforms with less than 120.degree. flat top value. They did not, however, consider the current rise delay in the incoming phase.
Murai et al., "Torque Ripple Improvement for Brushless DC Miniature Motors" IEEE Trans. on IA, Vol. 25, No. 3, pp. 441-450 (May/June 1989) developed pulse width modulation (PWM) methods and device conduction overlap periods to compensate for the difference in time required to turn-off the outgoing phase and fully turn-on the incoming phase. Their methods, however, resulted only in reduction of the torque ripple level, not in complete elimination of the commutation notching.
Berendsen et al., "Commutation Strategies for Brushless D.C. Motors: Influence on Instant Torque," Conf. Proc. of 1990 Applied Power Electronics Conf., pp. 394-400 (March 1990) proposed a neutral voltage feedback/compensation method to PWM regulate the machine phase currents during phase commutation and thus minimize to any degree desired (subject to PWM switching speed limitations) the commutation torque ripple. They also discussed methods useful when the back emf waveform has flat top periods of less than 120.degree. duration. In both cases, however, an extra sensor, to sense the neutral connection voltage, is required in addition to the normal phase current sensors.
Related co-pending U.S. patent application Ser. No. 07/939,123 discloses a quasi square wave brushless DC machine having five or more phases. The five (or more) phase design of the referenced machine yields an increased efficiency and/or torque density. Hence, the design holds great commercial and industrial promise. However, the existing solutions to the torque notch problem are inapplicable, inadequate, or excessive.
It would be greatly advantageous to provide a more practical method for smoothing the torque output of quasi square wave back EMF permanent magnet (PM) synchronous machine, including the five-phase PM machine of related co-pending U.S. patent application Ser. No. 07/939,123 as well as conventional three-phase PM machines, for all applications requiring smooth torque characteristics.